Thermal switch

ABSTRACT

A thermal switch has a pair of conductors with a longitudinally movable contact element engaging a side of at least one of the conductors and forming an electrical connection between the conductors. The contact element is biased against a thermally yieldable member such that when a predetermined temperature is reached the member yields and the element is moved longitudinally off an edge of the one conductor to open the electrical connection between the conductors.

United States Patent [191 Tyler et al.

THERMAL SWITCH Inventors: Hugh Jean Tyler; Denis G. Wolfe,

both of Santa Ana; Richard K. Mitts, Fullerton, all of Calif.

Robertshaw Controls Company, Richmond, Va.

Filed: July 13, 1973 Appl. No.: 378,843

Assignee:

U.S. Cl. 337/408, 337/414 Int. Cl. H0lh 37/76 Field of Search 337/408, 409, 407, 403,

337/413, 414, 401, 402, 186; ZOO/61.08, 163

References Cited UNITED STATES PATENTS 6/1930 Goetz 200/163 [111 3,820,050 June 25, 1974 3,291,945 12/1966 Merrill et al 337/403 3,304,396 2/1967 Hasson 337/407 3,500,279 3/1970 Malaspina 337/409 Primary Examiner-Harold Broome Attorney, Agent, or Firm-Anthony A. O'Brien [57] ABSTRACT 25 Claims, 11 Drawing Figures iATENIEDsuxzs m4 FIG. 5

ill/177M FIG 8 1 THERMAL SWITCH BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to thermal switches and, in particular, to a switch device containing a circuit connecting element biased against a member which yields at a specified temperature to open the circuit connecting element. Cooking ranges, water heaters, furnaces, and other appliances can be protected against overheating by such thermal switches.

2. Description of the Prior Art The prior art, as exemplified by US. Pat. Nos. 348,473, 2,997,562, 3,171,924, 3,291,945, and No. 3,519,972, contains many switch devices with contact elements biased against solid members which melt at a specified temperature to yield and allow the contact element to move out of engagement with one or more contacts to open a circuit. Many prior art thermal switching devices depend on maintaining contact force by the difference in force of two springs, one spring urging a movable contact against a stationary contact, and the other spring urging the movable contact and the one spring in the opposite direction against the meltable member; however, this construction doesnt allow the full force change potential or yield of the meltable element to be used in opening the circuit. Some prior art thermal switches depend on many small integral parts to perform a simple switching operation; however, some essential parts of such devices can be left out during assembly and the omission is undetectable by practical nondestructing testing. Also, some prior art thermal switches have been subject to being rendered inoperable and unsafe by denting or crushing a cover or case which prevents the movement of an element biased against a meltable member within the case. Further, contamination on contacts readily renders some prior art thermal switches inoperative or highly resistive.

SUMMARY OF THE INVENTION The invention is summarized in that a thermal switch includes a pair of conductors, one of the pair of conductors having a surface portion near an edge thereof, insulative means for supporting the one conductor with the conductor surface portion extending in a direction which has a substantial directional component parallel to an axis, a rigid member which becomes yieldable at a predetermined temperature, a movable electrically conductive contact, means connected to the contact for engaging the rigid member in a direction which prevents movement of the contact parallel to the axis from the one conductor surface portion past the edge, means for electrically connecting the contact to the other of the pair of conductors, biasing means for engaging the contact with the one conductor surface portion in a direction transverse to the axis to form an electrical connection between the one conductor and the contact, and biasing means for urging the contact toward the edge of the one conductor in a direction parallel to the axis with a force which exceeds the sum of the frictional force between the contact and the conductor and the force to which the rigid member yields at the predetermined temperature.

An object of the invention is to provide a simple thermally operated switch which is inexpensive and reliable.

Another object is to utilize all the potential force change of a meltable element to operate the switch mechanism.

It is also an object of the present invention to eliminate extremely small parts which can be left out of the assembly and which cannot be detected by practical non-destructive testing.

A further object of the invention is to eliminate the employment of thermal switch devices which have been rendered unsafe or inoperative by indentations caused by handling and installation.

One advantage of the invention is that wires used as connecting leads within an appliance are used as contacts in the thermal switch.

Another advantage is that switch resistance and resistance heating of a rigid member which becomes yieldable at a specified temperature is kept to a minimum.

Additional features of the invention include the employment of stripped ends of insulated wires as contact engaging surface portions; the provision of a movable contact element having a head portion with a pair of cantilever spring arms extending therefrom for engaging a pair of conductors; the provision of an inusulated support with a cavity having a pair of grooves for receiving and aligning a pair of contact engaging conductors; and the provision of an outer case which is made from a relatively brittle material to prevent denting.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation view in cross section of a thermal switch in accordance with the invention.

FIG. 2 is a side cross section view along lines 2-2 of FIG. 1.

FIG. 3 is a prospective view of a movable contact element of the thermal switch shown in FIG. 1.

FIG. 4 is a view similar to FIG. 1 but illustrating the movable contact element in an operated position.

FIG. 5 is an elevation view in cross section of a modified thermal switch in accordance with the invention.

FIG. 6 is a cross-sectional side view along line 66 of FIG. 5.

FIG. 7 is a perspective view of a movable contact element of the modified thermal switch of FIG. 5.

FIG. 8 is a view similar to FIG. 5 but illustrating the movable contact element in an operated position.

FIG. 9 is an elevation view in cross section of another modified thermal switch in accordance with the inventron.

FIG. 10 is an elevation view illustrating a step in the manufacture of the thermal switch of FIG. 9.

FIG. 11 is an elevation view in cross section of still another modified thermal switch in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As illustrated in FIGS. 1 and 2, the invention is embodied in a thermal switch which has a case 29 containing an insulator 21 supporting a pair of spaced conductors 22 and 24. A movable contact element, indicated generally at 26, engages and electrically connects the conductors 22 and 24. A spring 30 biases the movable contact element 26 against a rigid member 28 which yields at a predetemiined temperature, as illustrated in FIG. 4, such that the element 26 is moved out of engagement with the conductors 22 and 24 to break or open a circuit through the conductors 22 and 24.

The case is generally tubular with one end 32 being closed. A section of the tubular case 20 toward the closed end 32 is reduced in cross section and has the member 28 disposed therein against the closed end 32. The case 20 is formed from any suitable material, such as metal, by any suitable process, such as drawing.

The member 28 is a block of any suitable material or a device which is relatively rigid until heated to a specified temperature at which the member 28 becomes yieldable to the force of the spring 30. Suitable electrically nonconductive materials which are solid but become fluid or melt at specified temperatures are known and used and are commercially available from a variety of sources.

The insulator 21 is secured within the case 20 agains a shoulder 36 of the case 20 by inwardly deformed detents or tabs 38. The insulator 34 has a central cavity 40 formed in one end or a forward portion thereof with longitudinal grooves 42 and 43 in opposite wall portions of the cavity 40 aligned with respective holes 44 and 45 through a rear portion of the insulator 21. The cavity 40 is coaxial with the axis of the case 20 while the grooves 42 and 43 and holes 44 and 45 are parallel to the axis of the case 20. The holes 44 and 45 and the grooves 42 and 43 have cross-sectional sizes slightly larger than the cross-sectional sizes of the conductors 22 and 24 such that the conductors can be readily inserted longitudinally but are laterally retained. The grooves 42 and 43 have depths designed to leave substantial longitudinal surface portions of the conductors 22 and 24 exposed within the cavity 40. The insulator 21 is made from any suitable electrically insulating material, such as pressed steatite.

The conductors 22 and 24are wires with respective insulative coatings 48 and 50 and the exposed surface portions are sections of the wires from which the insulative coatings have been stripped. The stripped sections extend through the respective holes 44 and 45 into the grooves 42 and 43. The conductors 22 and 24 are illustrated as being single strand or solid core wires; however, wires with a multiple strand core can also be employed. The conductors 22 and 24 are selected to have sufficient rigidity that lengths of the.conductors extending from the holes 44 and 45 to end edges 46 and 47 in the cavity 40 are prevented from being bent by gravitational forces into engagement with each other.

A potting command 54 secures the insulated wires and conductors 22 and 24 and seals the open end of case 20. The potting compound 54 is typically a material which is thermal setting and which forms a good adhesive bond to the insulative coatings 48 and 50, conductors 22 and 24, the insulator 21, and the case 20. One suitable potting compound is Aremco 540-S-l.

The movable contact element 26, as shown in FIG.. 3, has a head 56 from which extend arms 58 and 60 which are cantilevered; i.e., supported only at or near one end. Tips or contacts 62 and 64 of the arms 58 and 60 having inwardly formed conical wedge portions 66 and 68. As illustrated in FIG. 1, a forward flat surface of the head 56 engages the face 33 of the member 28 and the arms 58 and 60 extend generally parallel to the axis of the case 20 into the cavity 40. The helical spring 30 is compressed between the forward end of the insulator 21 and the rear surface of the head 56. The head 56 is sufficiently smaller than the interior of the case 20 such that, when heated to the predetermined temperature, fluid material 28 is allowed to flow freely around the periphery of the head 56. The forward flat surface of the head 56 is sufficiently large to distribute the force from the spring 30 over a substantial portion of the face 33 of the member 28.

The tips 62 and 64 have a width and are spaced such that longitudinally extending surface portions adjacent opposite lateral edges of each of the wedge portions 66 and 68 engage the exposed longitudinal surface portions of both of the conductors 22 and 24. The wedge portions 66 and 68 are wedged between the conductors 22 and 24 to exert outward forces on the conductors to retain the conductors within the respective grooves 42 and 43.

At least the arms 58 and 60 of contact element 26 are made from a material which has sufficient elasticity or spring-like property to maintain the tips 62 and 64 in engagement with the conductors 22 and 24. At least the tips 62 and 64 of the arms 58 and 60 are made from an electrically conductive material to form an electrical connection between the conductors 22 and 24. Conveniently the contact element 26 is fabricated by stamping from a sheet of suitable conductive elastic metal or alloy and bending and forming the arms 58 and 60 and the tips 62 and 64. The tips 62 and 64 can be plated with contact metals or metals with low coefficients of friction, such as silver, gold or the like.

The arms 58 and 60 cooperate with each other by being elastically stressed in opposite pivotal directions about the head 56 to maintain each other in an elastic stressed condition and to prevent any tilting of the head 56 relative to the surface 33 of the member 28. The conductors 22 and 24 and the arms 58 and 60 are shown as being parallel to the axis of the casing 20; however, the conductors 22 and 24 and the arms 58 and 60 can extend in directions or at angles which have substantial directional or vectorial components parallel to the axis and the force of the spring 30 such that engaging forces between the conductors 22 and 24 and the tips 62 and 64 are transverse to the axis and the force of the spring 30.

The cross sections of the case 20, the insulator 21 and the head 56 perpendicular to the axis of the case, are non-circular or, as illustrated in FIGS. 2 and 3, somewhat rectangular or square. The size and shape of the head 56 relative to the case 20 is designed to maintain or guide the element 26 during assembly in substantially a predetermined relative rotative position about the longitudinal axis of the casing. The size and shape of the insulator 21 relative to the case 20 is such that the insulator 21 is also held or maintained in predetermined rotative position about the axis of the case, to assure that the conductors 22 and 24 in the grooves 42 and 43 of the insulator 21 have a predetermined relative rotative alignment with respect to the arms 58 and 60 and the tips 62 and 64 of the element 26.

In assembly of the thermal switch of FIGS. 1-3 the member 28 is inserted within the tubular case 20 against the end 32 and the contact element 26 is placed with the head 56 engaging the face 33 of the member 28. The spring 30 and the insulator 21 are assembled in the case 20 with the spring 30 compressed against the head 56. The insulator 21, the spring 30, the element 26 and the member 28 are secured in the case 20 by bending the tabs 38 inward. Suitable lengths of stripped sections 22 and 24 and the ends 46 and 47 of the wires 48 and 50 are inserted to a desired depth in the holes 44 and 43 and the grooves 42 and 43 either before or after the assembly of the insulator 21 in the case 20. The mating relationships of the head 58 and the insulator 21 with the case 20 is selected such that each of the tips 62 and 64 engages both of the conductors 22 and 24. The sloping surfaces of the conical wedge portions 66 and 68 allow the conductors 22 and 24 to spread the tips 62 and 64 as the conductors are inserted, and provide camming forces between the conductors 22 and 24 and the tips 62 and 64 to aid in the alignment of the element 26 relative to the conductors 22 and 24. The elastic force of the arms 58 and 60 urge the wedge portions 66 and 68 of the tips 62 and 64 between the conductors 22 and 24 to guide and hold the conductors in the grooves 42 and 43 during assembly.

. After sealing with the potting compound 54, the wires 48 and 50 are connected as a lead in an applicance in series with the appliance power source. The thermal switch is mounted in the appliance with the end 32 disposed in a position to be heated in the event the appliance becomes defective and overheated.

The operation of the thermal switch in response to an overheated condition is illustrated in FIG. 4. When the end 32 of the case 20 is exposed to a temperature which exceeds the melting point or softening point of the member 28, the member 28 becomes fluid allowing the spring 30 to push the head portion 56 toward the end 32 with fluid material 28 flowing around the periphery of the head 56. Movement of the contact element 26 slides the tips 62 and 64 off the edges 46 and 47 of the conductors 22 and 24, thus breaking or opening the circuit in which the wires 48 and 50 are connected. Upon repairing or correcting the cause of the overheating condition, the thermal switch is replaced by a new thermal switch.

One advantage of the thermal switch is that the electrical connection between the lateral edges of the tips 62 and 64 and the conductors 22 and 24 is made during assembly by a wiping action. The wiping action tends to eliminate many causes of failure resulting from dirt or contamination on the tips 62 and 64 or the conductors 22 and 24.

Another advantage is that the force on the tips 62 and 64 for engaging the tips with the conductors 22 and 24 is transverse to the longitudinal axis while the spring 30 biases the contact element 26 parallel to the longitudinal axis. Thus the spring 30 need only be designed to overcome the sum of the frictional forces between the tips 62 and 64 and the conductors 22 and 24 and the viscosity forces or resistance of the material 28 when it becomes fluid.

Additionally, the entire movement of the head 56 under the force of the spring 30 is used to withdraw or remove the contacting tips 62 and 64 from the conductors 22 and 24; thus the entire full force change potential of the yielding member 28 is used in opening the circuit.

Further, the major conductive paths between the conductors 22 and 24 are through each of the tips 62 and 64 shunting minor conductive paths through the head 56; thus resistance heating of the head 56 and the member 28 is kept to a minimum.

Still further, the thermal switch utilizes the stripped sections of wires as contacts, a single interconnecting contact element, a single spring, an insulator, and a case, all being relatively large with each part performing such a vital function that omission of a part during assembly is easily detectable; thus the thermal switch has the advantages of being less expensive and more reliable than prior thermal switches which depend upon many small intricate parts.

A modification of the thermal switch is illustrated in FIGS. 5-8. The same numerals are used in the modification to illustrate parts which are substantially similar in structure and/or function to the similarly identified parts of the embodiment shown in FIGS. 1-4, and such similarly identified parts for sake of brevity are not redescribed. The spring arm 58 and of the contact element 26 has tips 162 and 164 which have outwardly deformed portions 166 and 168. The tips 162 and 164 are designed to be inserted between the conductors 22 and 24 to engage the outward formed portions 166 and 168 with the exposed surface portions of the conductors 22 and 24. The elastic property of the spring arms 58 and 60 together with the shape of the outwardly formed portions 166 and 168 forces the spring arms 58 and 60 together at a fulcrum point 169 to engage and form an electrical connection therebetween. .The spring arms 58 and 60 urge the tips 162 and 164 to pivot in opposite directions about fulcrum point 169 into engagement with the conductors 22 and 24 and maintain the conductors 22 and 24 in the grooves 42 and 43. The contact at point 169 provides a short conductive path for current through the conductors 22 and 24 shunting the conductive path through the head 56.

As illustrated in FIG. 8 the modification operates in a manner similar to the embodiment of FIGS. l-4 in that, when the material 28 reaches a temperature where it becomes fluid, the head portion 56 is urged against the end 32 by the force of the spring 30 sliding the tips 162 and 164 off the edges 46 and 47 out of engagement with the conductors 22 and 24. Only the sum of the frictional force between the tips 162 and 164 and the conductors 22 and 24 and the viscosity forces of the material 28 when fluid need be overcome by the spring 30.

Another modification of the thermal switching device is illustrated in FIG. 9 wherein the same numbers are used to identify parts with substantially the same structure and/or function. The forward portion 201 of the insulator including the cavity 40 and the rear portion 203 of the insulator including the holes 44 and 45 are formed separately. The conductors 22 and 24 have crimped portions 205 formed at a predetermined distance from the ends 46 and 47 to engage the bottom of a recess 207 in the rear insulator portion 203 to accurately select the lengths of conductors 22 and 24 which are positioned within the cavity 40. The conductors 22 and 24 can be assembled in the rear insulator portion 203 and secured with a potting compound 209 previous to insertion in the case 20.

In the modification of FIG. 9, the case 20 is made of a fragile material, such as glass or metal which has been hardened to a point where it becomes brittle. In securing the insulator portions 201 and 203 within the case 20, a modified procedure illustrated in FIG. 10 is utilized. The tip of the case 20 containing the yieldable member 28 is immersed in a bath of cooling liquid. Thereafter the dimpled portion or detents 38 are formed by directing an intense source of heat such as a heliarc or plasma are against the case to form the detents 38. After sealing with the potting compound 54 the thermal switch is dipped into a suitable electrical insulative material, such as epoxy, to form a protective coating 211.

The use of a hardened or brittle case prevents denting of the case such as might occur in installation or handling procedures normally employed in manufacturing steps. In the event the case 20 is subjected to a force which would dent a nonbrittle case, the brittle case 20 will break rather than dent thus preventing the installation of a defective thermal switch which would not work due to a dent formed near the head 56.

A further modification of the thermal switch is illustrated in FIG. 11 wherein the end 32 is disposed at a position quite remote from the conductors 22 and 24. The spring arms 58 and 60 are elongated in the modification of FIG. 11 and are spot welded together at a point 301 adjacent the tips 62 and 64. Alternatively the tips 62 and 64 can be bifurcations of a rod extending from the head 56. The case 20 is substantially elongated with an inner tube or spacer 303 inserted between the end of the insulator 201 and the spring 30. The modification of FIG. '11 illustrates that the contact element 26 can be substantially elongated while maintaining the same relatively low resistance path path between the conductors 22 and 24 by the tips 62 and 64. The welded portion 301 ensures that the tips 62 and 64 still have substantial elasticity to insure the engagement with the conductors 22 and 24.

Since many variations, modifications and changes in detail may be made to the present embodiments, it is intended that all matter in the foregoing description and in the accompanying drawings be interrupted as illustrative and not in a limiting sense.

What is claimed is: 1. A thermal switch comprising a pair of conductors, one of the pair of conductors having a surface portion near an edge thereof,

insulative means for supporting the one conductor with the one conductor surface portion extending in a direction which has a substantial directional component parallel to an axis,

a rigid member which becomes yieldable at a predetermined temperature, a movable electrically conductive contact, means connected to the contact for engaging the rigid member in a direction which prevents movement of the contact parallel to the axis from the one conductor surface portion past the edge,

means for electrically connecting the contact to the other of the pair of conductors, biasing means for engaging the contact with the one conductor surface portion in a direction transverse to the axis to form an electrical connection between the one conductor and the contact, and

biasing means for urging the contact toward the edge of the one conductor in a direction parallel to the axis with a force which exceeds the sum of the frictional force between the contact and the conductor and the force to which the rigid member yields at the predetermined temperature.

2. A thermal switch as claimed in claim 1 wherein the one conductor surface portion is substantially parallel to the axis.

3. A thermal switch as claimed in claim 1 wherein the means connected to the contact for engaging the rigid member includes a head engaging the rigid member,

the biasing means for engaging the contact with the one conductor surface portion includes a cantilever spring arm extending from the head in a direction having a substantial directional component parallel to the axis, and

the contact includes a conductive tip on the spring arm biased by the spring arm against the one conductor surface portion.

4. A thermal switch as claimed in claim 1 wherein the one conductor is an insulated conductive wire,

and

the one conductor surface portion is a section at the end of the wire from which the insulation has been stripped.

5. A thermal switch as claimed in claim 4, wherein the insulative means includes a first portion having a cavity with a groove formed in the wall of the cavity for receiving the stripped section of the wire, and

the biasing means for engaging the contact with the stripped section urges the stripped section into the groove.

6. A thermal switch as claimed in claim 5 wherein the insulative means includes a second portion having a hole in alignment with the groove in the first portion of the insulating means, the wire extends through the hole and is supported by the second portion of the insulative means, and

the length of the wire extending from the hole into the cavity has sufficient rigidity to prevent any substantial bending from the force of gravity. 7. A thermal switch as claimed in claim 6 wherein there is included a case for supporting the rigid member in one end of the case and for supporting the insulating means near another end of the case,

the means connected to the contact for engaging the rigid member includes a head engaging the rigid member,

the biasing means for engaging the contact with the stripped section includes a first cantilever spring arm extending from the head in a direction having a substantial directional component parallel to the axis, and

the contact includes a conductive tip on the first spring arm biased by the spring arm against the stripped section of the wire.

8. A thermal switch as claimed in claim 7 wherein the rigid member includes a block of solid material which becomes fluid at the predetermined temperature, the solid block having a flat surface perpendicular to the axis;

the biasing means for urging the contact toward the one conductor edge includes a helical compression spring between the insulative means and the head; and

the head has a flat surface engaging the flat surface of the solid block.

9. A thermal switch is claimed in claim 7 wherein the biasing means for engaging the contact with the stripped section includes a second cantilever spring arm extending from the head in a direction having a substantial directional component parallel to the axis, and

means engaged by the second spring arm for maintaining the first spring arm in an elastic stressed condition to force the conductive tip transversely against the stripped section of the wire.

10. A thermal switch comprising a pair of conductors, one conductor having a surface portion;

insulative means having a groove for receiving and retaining the one conductor with the one conductor surface portion exposed;

a rigid member which becomes yieldable at a predetermined temperature;

a movable element including means for engaging the rigid member and an elastic cantilever arm with a conductive tip biased by the elastic arm into engagement with the one conductor surface portion,

means for electrically connecting the conductive tip to the other conductor, and

biasing means for urging the movable element against the rigid member such that when rigid member becomes yieldable the movable element and conductive tip are moved out of engagement with the one conductor surface portion.

11. A thermal switch as claimed in claim 10 where the conductors are insulated wires,

the one conductor surface portion is a section at the end of one wire having the insulation stripped therefrom, and

the insulative means includes a hole aligned with the groove for receiving the one wire longitudinally and retaining the one wire laterally.

12. A thermal switch comprising a pair of conductors each having a longitudinal surface portion near the edge thereof,

insulative means for supporting the conductors in spaced relationship with the longitudinal surface portions of the conductors extending in directions which have substantial directional components parallel to an axis,

a rigid member which becomes yieldable at a predetermined temperature,

a movable means for conducting electrical current,

means connected to the movable means for engaging the rigid member in a direction which prevents movement of the movable means parallel to the axis from the longitudinal surface portions past the edges,

means for biasing the movable means transverse to the axis into engagement against the longitudinal surface portions of both of the conductors to form an electrical connection between the conductors, and

means for biasing the movable means toward the edges of the conductors in a direction parallel to the axis with a force which exceeds the sum of the frictional force between the contact and the conductor and the force to which the rigid member yields at the predetermined temperatures.

13. A thermal switch as claimed in claim 12 wherein the means connected to the movable means for engaging the rigid member includes a head engaging the rigid member,

the means for biasing the movable means transverse to the axis includes a pair of cantilever spring arms extending from the head in directions which have portions and engage respective surface portions and each other.

16. A thermal switch as claimed in claim 12 wherein the pair of conductors are insulated wires with the insulation stripped from sections at ends of the wires to form the longitudinal surface portions,

the insulative means includes a first portion having a cavity with a pair of grooves each formed in wall portions of the cavity for receiving the respective stripped sections,

the means for biasing the movable means transverse to the axis also bias the stripped sections into the respective grooves.

17. A thermal switch as claimed in claim 16 wherein the means connected to the movable means includes a head engaging the rigid member,

the movable means for biasing the movable means transverse to the axis includes a pair of cantilever spring arms extending from the head in directions which have substantial directional components parallel to the axis, and

the movable means includes a pair of conductive tips on the respective ends of the spring arms engaging the stripped sections of the wires.

18. A thermal switch as claimed in claim 17 wherein the pair of grooves are formed in opposite wall portions of the cavity, and the conductive tips are wedge shaped and are wedged by the springs between the stripped sections of the wires. 19. A thermal switch as claimedin claim 17 wherein the pair of grooves are formed in opposite wall portions of the cavity, and the conductive tips are disposed between the stripped sections and are biased by the spring arms into engagement with the respective stripped sections and each other. 20. A thermal switch as claimed in claim 19 wherein the conductive tips engage each other at a fulcrum point and the spring arms bias the conductive tips in a pivotal relationship about the fulcrum point against the respective stripped sections.

21. A thermal switch comprising a pair of conductors,

a tubular case,

a block of rigid material which becomes fluid at a predetermined temperature secured at one end of the casing,

a movable head with a cross section perpendicular to the axis of the case slightly smaller than the cross section of the internal passageway of the case, means for biasing the head against the block,

means in the other end of the case operated by the movablehead for electrically connecting the pair of conductors until the predetermined temperature is reached and the head is allowed to be moved by the biasing means to electrically disconnect the pair of conductors, and

said tubular case being made of a brittle material which breaks upon impact rather than bending upon impact.

22. A thermal switch as claimed in claim 21 wherein the pair of conductors are insulated wires with the insulation stripped from sections at the ends of the wires,

there is included insulative means for supporting the stripped sections in spaced relationship longitudinally within the case,

the electrically connecting means includes a pair of cantilever spring arms extending longitudinally from the head and a conductive tip on at least one of the spring arms biased by the spring arms into engagement with the stripped sections to electrically connect the wires, and

the biasing means includes a helical compression spring between the head and insulative means.

23. A thermal switch comprising a tubular case closed at one end;

a block of rigid material which becomes fluid at a predetermined temperature disposed in the one end of the case;

a pair of insulated wires having the insulation stripped from sections at the ends of the wires; insulative means mounted toward the other end in the case having a central cavity with a pair of parallel grooves formed longitudinally relative to the 12 case in opposite wall portions of the cavity for receiving the stripped sections;

a movable conductive element having a head portion engaging the block, a pair of cantilever spring arms extending longitudinally from the head portion into the central cavity of the insulative means, and conductive tip portions on the respective spring arm portions biased by the spring arm portions transversely relative to the case against the stripped sections of the wires to form an electrical connection between the conductors; and

a compression spring between the insulator and the head portion for moving the movable element and the conductive tip portions from the central cavity when the solid material becomes fluidr 24. A thermal switch as claimed in claim 23 wherein 25. A thermal switch as claimed in claim 23 wherein the tubular case, the insulative means and the head portion of the movable element have mating noncircular cross sections perpendicular to the axis of the tubular case to align the conducting tip portions relative to the conductors. 

1. A thermal switch comprising a pair of conductors, one of the pair of conductors having a surface portion near an edge thereof, insulative means for supporting the one conductor with the one conductor surface portion extending in a direction which has a substantial directional component parallel to an axis, a rigid member which becomes yieldable at a predetermined temperature, a movable electrically conductive contact, means connected to the contact for engaging the rigid member in a direction which prevents movement of the contact parallel to the axis from the one conductor surface portion past the edge, means for electrically connecting the contact to the other of the pair of conductors, biasing means for engaging the contact with the one conductor surface portion in a direction transverse to the axis to form an electrical connection between the one conductor and the contact, and biasing means for urging the contact toward the edge of the one conductor in a direction parallel to the axis with a force which exceeds the sum of the frictional force between the contact and the conductor and the force to which the rigid member yields at the predetermined temperature.
 2. A thermal switch as claimed in claim 1 wherein the one conductor surface portion is substantially parallel to the axis.
 3. A thermal switch as claimed in claim 1 wherein the means connected to the contact for engaging the rigid member includes a head engaging the rigid member, the biasing means for engaging the contact with the one conductor surface portion includes a cantilever spring arm extending from the head in a direction having a substantial directional component parallel to the axis, and the contact includes a conductive tip on the spring arm biased by the spring arm against the one conductor surface portion.
 4. A thermal switch as claimed in claim 1 wherein the one conductor is an insulated conductive wire, and the one conductor surface portion is a section at the end of the wire from which the insulation has been stripped.
 5. A thermal switch as claimed in claim 4, wherein the insulative means includes a first portion having a cavity with a groove formed in the wall of the cavity for receiving the stripped section of the wire, and the biasing means for engaging the contact with the stripped section urges the stripped section into the groove.
 6. A thermal switch as claimed in claim 5 wherein the insulative means includes a second portion having a hole in alignment with the groove in the first portion of the insulating means, the wire extends through the hole and is supported by the second portion of the insulative means, and the length of the wire extending from the hole into the cavity has sufficient rigidity to prevent any substantial bending from the force of gravity.
 7. A thermal switch as claimed in claim 6 wherein there is included a case for supporting the rigid member in one end of the case and for supporting the insulating means near another end of the case, the means connected to the contact for engaging the rigid member includes a head engaging the rigid member, the biasing means for engaging the contact with the stripped section includes a first cantilever spring arm extending from the head in a direction having a substantial directional component parallel to the axis, and the contact includes a conductive tip on the first spring arm biased by the spring arm against the stripped section of the wire.
 8. A thermal switch as claimed in claim 7 wherein the rigid member includes a block of solid material which becomes fluid at the predetermined temperature, the solid block having a flat surface perpendicular to the axis; the biasing means for urging the contact toward the one conductor edge includes a helical compression spring between the insulative means and the head; and the head has a flat surface engaging the flat surface of the solid block.
 9. A thermal switch is claimed in claim 7 wherein the biasing means for engaging the contact with the stripped section includes a second cantilever spring arm extending from the head in a direction having a substantial directional component parallel to the axis, and means engaged by the second spring arm for maintaining the first spring arm in an elastic stressed condition to force the conductive tip transversely against the stripped section of the wire.
 10. A thermal switch comprising a pair of conductors, one conductor having a surface portion; insulative means having a groove for receiving and retaining the one conductor with the one conductor surface portion exposed; a rigid member which becomes yieldable at a predetermined temperature; a movable element including means for engaging the rigid member and an elastic cantilever arm with a conductive tip biased by the elastic arm into engaGement with the one conductor surface portion, means for electrically connecting the conductive tip to the other conductor, and biasing means for urging the movable element against the rigid member such that when rigid member becomes yieldable the movable element and conductive tip are moved out of engagement with the one conductor surface portion.
 11. A thermal switch as claimed in claim 10 where the conductors are insulated wires, the one conductor surface portion is a section at the end of one wire having the insulation stripped therefrom, and the insulative means includes a hole aligned with the groove for receiving the one wire longitudinally and retaining the one wire laterally.
 12. A thermal switch comprising a pair of conductors each having a longitudinal surface portion near the edge thereof, insulative means for supporting the conductors in spaced relationship with the longitudinal surface portions of the conductors extending in directions which have substantial directional components parallel to an axis, a rigid member which becomes yieldable at a predetermined temperature, a movable means for conducting electrical current, means connected to the movable means for engaging the rigid member in a direction which prevents movement of the movable means parallel to the axis from the longitudinal surface portions past the edges, means for biasing the movable means transverse to the axis into engagement against the longitudinal surface portions of both of the conductors to form an electrical connection between the conductors, and means for biasing the movable means toward the edges of the conductors in a direction parallel to the axis with a force which exceeds the sum of the frictional force between the contact and the conductor and the force to which the rigid member yields at the predetermined temperatures.
 13. A thermal switch as claimed in claim 12 wherein the means connected to the movable means for engaging the rigid member includes a head engaging the rigid member, the means for biasing the movable means transverse to the axis includes a pair of cantilever spring arms extending from the head in directions which have substantial directional components parallel to the axis, and the movable means includes a pair of conductive tips on the respective ends of the spring arms engaging the conductor surface portions.
 14. A thermal switch as claimed in claim 13 wherein each of the conductive tips engages both conductor surface portions.
 15. A thermal switch as claimed in claim 13 wherein the conductive tips are between the conductor surface portions and engage respective surface portions and each other.
 16. A thermal switch as claimed in claim 12 wherein the pair of conductors are insulated wires with the insulation stripped from sections at ends of the wires to form the longitudinal surface portions, the insulative means includes a first portion having a cavity with a pair of grooves each formed in wall portions of the cavity for receiving the respective stripped sections, the means for biasing the movable means transverse to the axis also bias the stripped sections into the respective grooves.
 17. A thermal switch as claimed in claim 16 wherein the means connected to the movable means includes a head engaging the rigid member, the movable means for biasing the movable means transverse to the axis includes a pair of cantilever spring arms extending from the head in directions which have substantial directional components parallel to the axis, and the movable means includes a pair of conductive tips on the respective ends of the spring arms engaging the stripped sections of the wires.
 18. A thermal switch as claimed in claim 17 wherein the pair of grooves are formed in opposite wall portions of the cavity, and the conductive tips are wedge shaped and are wedged by the springs between the stripped sections of the wires.
 19. A thermal switCh as claimed in claim 17 wherein the pair of grooves are formed in opposite wall portions of the cavity, and the conductive tips are disposed between the stripped sections and are biased by the spring arms into engagement with the respective stripped sections and each other.
 20. A thermal switch as claimed in claim 19 wherein the conductive tips engage each other at a fulcrum point and the spring arms bias the conductive tips in a pivotal relationship about the fulcrum point against the respective stripped sections.
 21. A thermal switch comprising a pair of conductors, a tubular case, a block of rigid material which becomes fluid at a predetermined temperature secured at one end of the casing, a movable head with a cross section perpendicular to the axis of the case slightly smaller than the cross section of the internal passageway of the case, means for biasing the head against the block, means in the other end of the case operated by the movable head for electrically connecting the pair of conductors until the predetermined temperature is reached and the head is allowed to be moved by the biasing means to electrically disconnect the pair of conductors, and said tubular case being made of a brittle material which breaks upon impact rather than bending upon impact.
 22. A thermal switch as claimed in claim 21 wherein the pair of conductors are insulated wires with the insulation stripped from sections at the ends of the wires, there is included insulative means for supporting the stripped sections in spaced relationship longitudinally within the case, the electrically connecting means includes a pair of cantilever spring arms extending longitudinally from the head and a conductive tip on at least one of the spring arms biased by the spring arms into engagement with the stripped sections to electrically connect the wires, and the biasing means includes a helical compression spring between the head and insulative means.
 23. A thermal switch comprising a tubular case closed at one end; a block of rigid material which becomes fluid at a predetermined temperature disposed in the one end of the case; a pair of insulated wires having the insulation stripped from sections at the ends of the wires; insulative means mounted toward the other end in the case having a central cavity with a pair of parallel grooves formed longitudinally relative to the case in opposite wall portions of the cavity for receiving the stripped sections; a movable conductive element having a head portion engaging the block, a pair of cantilever spring arms extending longitudinally from the head portion into the central cavity of the insulative means, and conductive tip portions on the respective spring arm portions biased by the spring arm portions transversely relative to the case against the stripped sections of the wires to form an electrical connection between the conductors; and a compression spring between the insulator and the head portion for moving the movable element and the conductive tip portions from the central cavity when the solid material becomes fluid.
 24. A thermal switch as claimed in claim 23 wherein the insulative means has a portion with parallel holes extending longitudinally for receiving and supporting the wires in parallel spaced relationship with lengths of the wires extending from the holes into the central cavity, and the rigidities of the lengths of the wires are selected to prevent the stripped sections of the wires from being bent by gravitational forces into engagement.
 25. A thermal switch as claimed in claim 23 wherein the tubular case, the insulative means and the head portion of the movable element have mating non-circular cross sections perpendicular to the axis of the tubular case to align the conducting tip portions relative to the conductors. 